1. Field of the Invention
The present invention relates to a zoom lens, which is suitable for an image pickup lens in an image pickup apparatus, such as a video camera, a surveillance camera, a digital still camera, a broadcasting camera, and a film-based camera.
2. Description of the Related Art
An image pickup optical system used for an image pickup apparatus using a solid-state image sensing device, such as a video camera, a surveillance camera, and a digital still camera, is required for a small zoom lens having a wide angle of view, a high zoom ratio, and a high optical performance. One known zoom lens that satisfies these requirements is a four-unit zoom lens that includes, in order from the object side to the image side, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, a third lens unit having a positive refractive power, and a fourth lens unit having a positive refractive power. In this type, a conventional, so-called rear focus type four-unit zoom lens is configured to vary a magnification by moving the second lens unit, to correct an image fluctuation associated with the magnification variation using the fourth lens unit, and to provide focusing (U.S. Pat. No. 6,084,722 and U.S. Patent Application, Publication No. 2008/0310033).
In general, in order to makes the zoom lens smaller and the zooming ratio higher, a larger refractive power of each lens unit is effective. However, simply making larger the refractive power of each lens unit would enlarge the aberrational fluctuations associated with zooming and cause difficulties to obtain good optical performance over the entire zoom range.
In order for the above rear focus type four-unit zoom lens to have a small overall system, a wide angle of view, and a high zoom ratio, it is important to properly set a negative refractive power and a lens configuration of the magnification-varying second lens unit. When the lens shape, the configuration, the refractive power, etc. of each lens in the second lens unit are improper, it becomes difficult to provide a small overall system with a wide angle of view and a high zoom ratio. In addition, it becomes difficult to obtain a high optical performance since fluctuations of a variety of aberrations associated with zooming increase. It is also important to properly set the refractive power of each lens unit for the wide angle of view in the small overall system while the predetermined zoom ratio is maintained.